1. Field of the Invention
The invention relates in general to a touch system, and more particularly, to a technique for enhancing the accuracy in sensing results of a border region of a touch panel.
2. Description of the Related Art
Operating interfaces of recent electronic products are becoming increasingly user-friendly and intuitive. For example, through a touch screen, a user can directly interact with applications as well as input messages/texts/patterns with fingers or a stylus, thus eliminating complications associated with other input devices such as a keyboard or buttons. In practice, a touch screen usually comprises a touch panel and a display provided at the back of the touch panel. According to a touch position on the touch panel and a currently displayed image on the display, an electronic device determines an intention of the touch to execute corresponding operations.
Existing capacitive touch sensing techniques can be roughly categorized into self-capacitive and mutual-capacitive types. Compared to mutual-capacitive touch panels, self-capacitive touch panels can be implemented through a single-layer electrode with a simpler manufacturing process and lower costs, and thus prevail in many entry-level electronic products.
FIG. 1 shows an exemplary self-capacitive touch panel. A sensing area 100 demarcated by a dotted frame includes multiple electrodes, e.g., electrodes 11, 12, 14, 15 and 17. The electrodes are equal in width, and appear similar to right triangles. Since a sensor for sensing capacitance change in the electrodes is expensive, current touch panels are generally designed to have multiple electrodes sharing a same sensor in order to be cost-effective. As shown in FIG. 1, the electrodes 11 and 12 are connected to a first upper sensor 13, and the electrodes 14 and 15 are connected to a first lower sensor 16. In other words, the capacitance change sensed by the first upper sensor 13, rather than being capacitance change of either of the electrodes 11 and 12, is a sum of the capacitance changes occurring at the electrodes 11 and 12. In FIG. 1, the capacitance changes sensed by 2*N number of sensors (N number of upper sensors and N number of lower sensors, each having a designated number and serves as an ith sensor, where i=1 to 2N) are transmitted to a controller (not shown) for the controller to determine a position of a user touch. The controller may calculate an X-coordinate x of the user touch position according to the equation below:
                    x        =                                            ∑                              i                =                1                                            2                ⁢                N                                      ⁢                                                  ⁢                          (                                                C                  i                  *                                ⁢                                  X                  i                                            )                                                          ∑                              i                =                1                                            2                ⁢                N                                      ⁢                                                  ⁢                          C              i                                                          equation        ⁢                                  ⁢                  (          1          )                    
In equation (1), i is an integral index ranging from 1 to 2N, Ci represents the capacitance change sensed by the ith sensor, and Xi represents coordinates of a common center of gravity of the electrodes connected to the ith sensor. Taking the first upper sensor 13 as an example, the corresponding coordinates Xi of a center of gravity are a position of a common center of gravity of the two electrodes 11 and 12 (between the electrodes 11 and 12).
The controller may further calculate a Y-coordinate y of the user touch position according to the equation below:
                    y        =                              (                                                            r                  *                                      C                    U                                                  -                                  C                  D                                                            r                -                1                                      )                    *                      (                          L                              C                T                                      )                                              equation        ⁢                                  ⁢                  (          2          )                    
In equation (2), r represents a predetermined value associated with a size of the electrodes, CU represents a total capacitance change sensed by the N upper sensors, CD represents a total capacitance change sensed by the N lower sensors, CT is a sum of CU and CD, and L represents a height with respect to the Y direction. As shown in FIG. 2, the electrodes are in fact tall and narrow trapezoids with an upper side having a width dxs and a lower side having a width dxl. The value r is defined as:
                    r        ≡                              dxl            +                          0.9              *              dxx                                            dxs            +                          0.9              *              dxx                                                          equation        ⁢                                  ⁢                  (          3          )                    
In equation (3), dxx represents a width of a gap between every two adjacent electrodes.
However, the touch panel in FIG. 1 encounters a great challenge—errors in the sensing results of the left and right borders are extremely large. The electrodes 11, 12, 14, 15 and 17 are again depicted in FIGS. 3A to 3C to explain reasons behind such problem.
When a user touch takes places at a position within a dotted circle 21 in FIG. 3A, only the electrode 14 is affected, meaning that only the first lower sensor 16 senses the capacitance change. However, substantially errors exist in the coordinates x and y calculated based on the capacitance change sensed by the first lower sensor 16. It is seen from equation (2) that, a dominant basis for determining the coordinate y is a relation between the two capacitance changes CU and CD. When only the electrode 14 is affected, the capacitance change CU is zero regardless of what the Y coordinate of the user touch position is, such that the coordinate y obtained according to equation (2) is a negative value. It is obvious that the above calculation does not correctly respond the Y coordinate of the touch position.
On the other hand, although the capacitance change sensed by the first lower sensor 16 is chiefly contributed by the electrode 14, instead of utilizing the center of gravity of the electrode 14, the controller however utilizes the position of the common center of gravity (denoted as P1) of the electrodes 14 and 15 to represent the position corresponding to the capacitance change sensed by the first lower sensor 16. Accordingly, the coordinate x obtained is slightly shifted to the right from the actual position of the circle 21. Referring to FIG. 3B, when the circle 21 is not located within the border region of the sensing area 100, the left half of the circuit 21 theoretically triggers another electrode 31 (a virtual electrode represented by a dotted triangle), to further provide a capacitance change that shifts a coordinate x calculated according to equation (1) to the left (i.e., a real X coordinate making the coordinate x closer to the circle 21). In other words, in the border region, due to the lack of a balancing value contributed by the virtual electrode 31, and the foregoing center of gravity P1 is located by a great distance from the real X coordinate of the circle 21, the coordinate x calculated by the controller may consequently contain a substantial error.
Similarly, when a user touch takes place at a position within a dotted circle 22 in FIG. 3C (i.e., the rightmost of the sensing area 100), only the electrode 17 is affected. Under the circumstances, only the Nth upper sensor senses the capacitance change, such that a substantially error also exists in the coordinates of the touch position calculated accordingly. It is seen from equation (2) that, when only the electrode 17 is affected, the capacitance change CD is zero regardless of what the Y coordinate of the user touch position is, such that the coordinate y obtained according to equation (2) is a constant value approximating L. It is obvious that the above calculation approach does not correctly represent the actual touch position.
The above issue of sensing errors in the border regions likely leads the controller to misjudge a real intention of a user touch to further trigger incorrect operation result. However, if the left and right border regions of the sensing area 100 are neglected to avoid the above issue, hardware costs are wasted.